In the ignition system of an spark ignited internal combustion engine, high voltage is applied by ignition cables to spark plugs. Energy is supplied from a battery to build up energy in the magnetic field of an ignition coil. Breaker points are opened and closed by operation of a cam shaft driven by the engine to control the flow of current to the coil from the battery. Upon interruption of the flow, the ignition coil produces a high voltage across the gap in a respective spark plug selected by the distributor to cause ignition in a respective cylinder as the magnetic field in the ignition coil collapses. The high voltage breaks down the dielectric in the gap, resulting in a spark that ignites the air-fuel mixture. The sparks are accompanied by violent surges of current in the ignition cable. Unless suppression means are provided, the ignition cable acts as an antenna and radiates a broad spectrum of frequencies caused by the sparking, causing interference with radio reception and with the proper operation of other electronic equipment. The FCC requires automobile manufacturers to meet SAE standards for allowable automobile electrical noise. The ignition system contributes a significant amount of this electrical noise, and, therefore, it is important that an ignition cable with good suppression means be used in this system.
Some ignition cable designs have suppressed interfering frequencies by using semiconductive cables or high resistance cables that attenuate interfering frequency currents. A drawback of these cables is that they also offer high resistance to the desired ignition current wasting useful power and inhibiting the sparking current. Furthermore, the current attenuation causes cable heating that results in premature aging, oxidation, and corrosion.
Rimsha, U.S. Pat. No. 3,454,907, discloses a radio frequency attenuating cable that preferentially conducts direct current. The Rimsha cable has an inner core made of copper clad with a cylindrical conductor of nickel. About this inner core is wound a high permeability metal wire which is heat fused to the nickel cladding. The object of this conductor design is to attenuate high frequencies arising from outside the system, as to isolate an electroexplosive device from an electromagnetic field as might arise from a nuclear explosion. Direct current is preferentially passed through the more conductive inner core while alternating current is crowded to outside surface by the skin effect. Skin effect occurs whenever alternating current is applied to a conductor, and the crowding increases with increases in frequency. The skin effect results from the greater impedance of the interior of the conductor with increase in frequency, occasioned by the greater inductance of the interior. As disclosed in Rimsha, the effective resistance of a conductor increases with frequency due to the skin effect, as the high frequency current is crowded into a smaller cross section. In Rimsha, the alternating current is crowded to the outer high permeability layer where it is attenuated. However, being metallic, the layer provides but limited damping.